As the semicolon acts as delimiter for mulit-value-fields for WMP could you check if you have got now 2 GENRE fields (open the extended tags dialogue for this: Alt-T)
It seems some sort of glitch may have happened earlier today, because I am now unable to replicate the problem myself (the source files have had some tagging changes since I posted).
I think I should apologise for this one and withdraw (and ask for the thread to be closed).
(I have now taken the precaution of reconfirming my other post about multilingual lyrics: that remains reproducible).